Transparent silica glass luminescent material and process for producing the same

ABSTRACT

It is an object of the present invention to provide a light emitting device of the next generation optical device having a broad emission property that a width at half maximum of an emission spectrum is large in a wavelength range of visible light and capable of recognizing white light emitting by photoluminescence (PL). In a baking process for baking a pressure molding formed by pressure molding of silica fine particles such as fumed silica, a baking temperature is made as not more than 1000° C., hydroxyl groups of the silica fine particles are sufficiently subjected to dehydration condensation reaction so that the particles becomes transparent, a structural defect generated in the process is held without being relaxed, and thus a silica glass is generated. The silica glass is employed as a fluorescent substance.

TECHNICAL FIELD

The present invention relates to a transparent silica luminescentmaterial and method for manufacturing the same, more specifically, itrelates to a transparent silica glass which is generated from silicafine particles, has emission property broad in a wavelength range ofvisible light and is applicable to white light emitting devicematerials.

BACKGROUND ART

In recent years, improvements in a visible short wavelength lightemitting diode (LED) of a nitride semiconductor type have been advancedand a white LED using the diode has been developed in place ofconventional illumination fixtures such as an electric bulb andfluorescent lamp. When the white LED is employed as illumination, thereare merits that: power is saved, running cost is lowered, safety isenhanced and a life is lengthened, compared to an incandescent lamp orthe fluorescent lamp; and it is unnecessary to use a toxic substancesuch as mercury as the fluorescent lamp.

When the white LED is realized, there are some choices. That is whylight color of an LED becomes unique to a semiconductor crystal used foran LED chip by depending on a band gap and light color of a general LEDbecomes a single color such as red, green or blue. As a means forrealizing the white LED, there is a means for gathering the red LED,green LED and blue LED and making all of the LED emit light at the sametime. Further, the white LED is realized by combination of electroluminescence (EL) and photoluminescence (PL) and with a fluorescentsubstance using the blue LED and a rare earth element, or the like.

The white LED is realized with the fluorescent substance using the blueLED and the rare earth element as described above. Therefore, processingof the white LED becomes complicated by use of the rare earth elementand problems are pointed out in terms of resource amounts and costs.Thus, a material for the next generation optical device is requiredwhich does not impact the environment at the time of disposal, of whichmanufacturing process is simpler than that of the conventional white LEDand which can meet the requirements of low-cost, energy savings andpreservation of the environment. (Patent Document 1 and Non-PatentDocument 1)

A sintering process of fine particles is disclosed in the Documentsregarding the silica glass as listed below, but a sintering processregarding a transparent silica glass is not disclosed therein. Further,emission property (white light emitting) are not disclosed therein.(Patent Documents Nos. 2 to 4 and Non-Patent Documents Nos. 2 to 4)

-   [Patent Document 1]: Japanese Published Unexamined Patent    Application No. 2001-156336-   [Patent Document 2]: Japanese Published Unexamined Patent    Application No. H02-133329-   [Patent Document 3]: Japanese Published Unexamined Patent    Application No. 2002-211935-   [Patent Document 4]: Japanese Published Unexamined Patent    Application No. H01-201664-   [Non-Patent Document 1]: MITSUBISHI CABLE INDUSTRIES, LTD. REPORT p.    35 to p. 40 (July, 2002)-   [Non-Patent Document 2]: G. V. Chandrashekhar; Mat. Res. Soc. Symp.    Proc. Vol. 73, p. 705 to p. 710 “DIELECTRIC PROPERTIES OF SOL-GEL    SILICA GLASSES”-   [Non-Patent Document 3]: Hiroshi Suzuki; Japan Ceramics Association    Scientific Paper Vol. 100, No. 3, p. 272 to p. 275 (March 1992)    “Fine structure control of porous silica glass using mono dispersion    sphere-shaped silica particles)-   [Non-Patent Document 4]: R. Clasen; Glastech. Ber. Vol. 61, No. 5,    pp. 119-126 (May 1988) “Preparation of glass and ceramics by    sintering colloidal particles deposited from the gas phase”

DISCLOSURE OF THE INVENTION

[The Problem to be Solved]

It is an object of the present invention to provide a light emittingdevice of the next generation optical device capable of recognizingwhite light emitting by photoluminescence (PL). That is, the object isto develop an light emitting device having a broad emission propertythat a half bandwidth of an emission spectrum is large in a wavelengthrange of visible light, the emission property differing from a featureof an LED that a half bandwidth of an emission spectrum is small andsingle color property is high.

As a result of diligent research for a defect generation process of anamorphous structure of a silica glass by the inventors, the inventorsfound that a transparent silica glass manufactured with use of silicafine particles under a specific baking temperature condition has thebroad emission property, in which the half bandwidth of the emissionspectrum is large in the wavelength range of visible light, andcompleted the present invention.

[Means for Solving the Problem]

In the present invention, a pressure molding formed by pressure moldingof silica fine particles is baked under a temperature condition that astructural defect is generated and held without being relaxed, and thusa transparent silica glass having a white emission property ismanufactured.

In a general method for manufacturing a silica glass, a high temperaturecondition of 1800° C. or more is required for heating and fusion ofsilica fine particles generated by heating and fusion of quartz crystalsor combustion of silicon tetrachloride under an oxygen-hydrogen flame.

On the other hand, in a method for manufacturing a silica glassaccording to the present invention, the silica glass is manufactured bysolid phase reaction of silica fine particles without melting. Morespecifically, a pressure molding is baked under a lower temperaturecondition than the melting temperature after pressure molding with useof a reaction between surfaces of chemically active silicas, and thusthe silica glass is manufactured with the structural defect generatedand held without being relaxed.

Here, the pressure molding with use of a reaction between surfaces ofchemically active silicas means that the silica fine particles arepressure molded and the pressure molding is formed. Moreover, it isdifficult to pressure mold the silica fine particles and form thepressure molding at a pressure of 5 MPa or less.

A baking process is performed at a temperature in a range of 500-1400°C. and in a time range of 1 minute to 300 hours so that the structuraldefect is generated and held.

The reason for the baking at a lower temperature than the meltingtemperature of the general silica glass and for a long time will beexplained below.

The silica fine particle usually contains a great number of hydroxylgroups (silanol). A defect structure that a silicon-oxygen bond iscleaved can be induced in a glass structure with use of a process forremoving the hydroxyl groups by dehydration condensation reaction. Atemperature of 200° C. or more is generally required to subject thesilica glass fine particles to the dehydration reaction. Further, atemperature of 500° C. or more is required so that the hydroxyl groupsbetween the silica glass fine particles are sufficientlydehydration-condensed.

However, when the pressure molding is baked at a temperature higher than1400° C., atom movement is actively performed, and therefore the defectgenerated by the dehydration condensation reaction of the hydroxylgroups is relaxed.

Thereupon, a baking temperature of the pressure molding is made to rangefrom 500° C. to 1400° C. so that the defect generated in the dehydrationcondensation process of the hydroxyl groups of the silica fine particlescan be held without being relaxed.

The baking process is performed in a time range of 1 minute to 300 hoursso that the dehydration condensation reaction of the hydroxyl groups ofthe silica fine particles is sufficiently performed. A water content ofthe general silica glass ranges from 300 ppm to 500 ppm. The baking timein a range of 1 minute to 300 hours is required in accordance with thebaking temperature so that a water content of the silica glass accordingto the present invention is made to range from 300 ppm to 500 ppm.

In particular, the transparent silica glass that the baking is performedat a temperature near 980° C. and in a time range of 120 to 200 hourshas a broad spectrum property that ranges the whole wavelength range ofvisible light as a peak at a wavelength of 520 nm and that a full widthat half maximum (FWHM) is approximately 200 nm, and further has a whiteemission property that a photoluminescence (PL) intensity is so highthat observation by the naked eye is possible.

As the silica fine particle used for the method for manufacturing thetransparent silica glass according to the present invention, fumedsilica is employed which is artificial amorphous silicon dioxide and isa high-purity super fine particle having a particle size of several nmto tens nm. That is why the solid phase reaction is smoothly performedbecause of the highly reactive surface of the fumed silica.

The particle size of the fumed silica is 1 to 100 nm, desirably 5 nm to100 nm and more desirably 5 nm to 50 nm. That is why the structuraldefect is likely to be generated when the particle size is small.Moreover, a particle size of fumed silica on the current market is 7 nmto 50 nm.

When the baking is performed at a temperature in a range of 1000° C. to1400° C. and in a time range of several minutes to 100 hours, thetransparent silica glass can be obtained which has a first emission peakin a wavelength of 400 nm to 500 nm, a second emission peak in awavelength of 650 nm and emission property broad in the wavelength rangeof visible light in a spectrum of the photoluminescence (PL). Thetransparent silica glass has a reddish white emission property by anemission peak at a wavelength of 650 nm.

When the silica fine particles are mixed with inorganic materialparticles having semi-conductivity and/or conductivity to be pressuremolded and baked, the defect is likely to be generated and a silicaglass having a red type emission property can be obtained. For example,when the silica fine particles are mixed with carbon, silicon or thelike, a pink silica glass having a red emission property is generated.

[The Effect of the Invention]

A transparent silica glass luminescent material according to the presentinvention has an effect that a width at half maximum of an emissionspectrum is large in a wavelength range of visible light and a broademission is performed. Further, a method for manufacturing thetransparent silica glass luminescent material according to the presentinvention is a simple manufacturing process constituted by only pressuremolding and baking, and the method has an effect that the transparentsilica glass luminescent material can be easily manufactured at a lowbaking temperature.

Further, the transparent silica glass luminescent material according tothe present invention is excellent in endurance and has an effect thatemission property does not change over a long time (a few months).

The possibility is high that the transparent silica glass luminescentmaterial is made practicable as a fluorescent material such as a whiteLED owing to the above-described emission property and manufacturingprocess.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Best modes for carrying out the present invention will be explainedbelow with reference to the accompanying drawings.

First Embodiment

A method for manufacturing a transparent silica glass according to thepresent invention includes a pressurizing process for pressure moldingsilica fine particles and forming a pressure molding and a bakingprocess for baking the pressure molding. The baking is performed under atemperature condition that a structural defect is generated and heldwithout being relaxed. An embodiment of the manufacturing method will beexplained below.

As the silica fine particle used for manufacturing the transparentsilica glass, fumed silica, etc., is employed. Silicon tetrachloride gasis oxidized and hydrolyzed with a flame of 1100° C. to 1400° C. that amixed gas of hydrogen and oxygen is burned, and thus the fumed silica ismanufactured. The fumed silica is a sphere-shaped super fine particlethat an average size of first particles is approximately 10 nm, and themain ingredient of the fumed silica is amorphous silicon dioxide (SiO₂).The fumed silica is a super fine particle and is manufactured by rapidcooling, thereby having a surface structure of high chemical activity.

It is desirable that an average size of first particles of fumed silicaused for manufacturing of transparent silica glass is from several nm totens nm. That is why, for example, in fumed silica of which an averagesize of first particles exceeds 100 nm, a chemical activity force of thesurface is small and the fusion bond effect of the fumed silica duringthe pressure load as described below becomes small and therefore thefumed silica is unsuitable for the manufacturing of transparent silicaglass.

Moreover, the fumed silica actually used is as follows.

-   Maker: Sigma, St. Louis, Mo., USA-   Type number: S 5130-   Particle size: 7 nm

Furthermore, each analysis value of impurities of standard fumed silicais indicated below.

-   Al203: 0.001% or less, Fe203: 0.0001% or less, Ti02: 0.001% or less

Next, the pressurizing process for pressure molding the silica fineparticles and forming the pressure molding will be explained. FIG. 1shows a conceptual diagram of the pressure molding of the silica fineparticles. For example, the fumed silica is weighed by approximately 0.3g and pressurized for 3 minutes at 529 MPa (150 kN to a pellet area2.835 cm²) with use of a high-pressure molder, and thus the pellet ofthe silica glass can be manufactured.

Next, the baking process for baking the pressure molding will beexplained. The above-described pressure molding of the fumed silica ismade to be placed into an electric furnace to be baked under atmosphericpressure. For example, a baking temperature is made as 1000° C. or lessand a baking time is made as 100 hours or more. That is why a part ofthe structural defect is relaxed and the defect cannot be sufficientlyheld when the baking temperature exceeds 1000° C. Further, that is whydehydration condensation of hydroxyl groups is insufficient and a defectof a sufficient concentration cannot be induced when the baking time is100 hours or less even though the baking temperature is 1000° C. orless. As a tendency, the baking time becomes shorter as the bakingtemperature becomes higher, and the baking time becomes longer as thebaking temperature becomes lower.

However, an optimum baking time depends on a baking temperaturecondition. That is why when the baking temperature is high, the defectis likely to be generated by a rapid advance of a dehydrationcondensation reaction of the hydroxyl groups, on the contrary, thedefect is likely to be relaxed. Therefore, when the baking temperatureis high, the baking time is required to be shortened.

As a result of diligent research, the inventors found that a baking timeof 168 hours is desirable when the baking temperature is 980° C.

Here, the muffle furnace (Type number: KDF S70, Maker: Denken Co., Ltd.)is employed for baking of the pellet of the silica glass.

Emission property of the transparent silica glass manufactured by theabove-described manufacturing method will be explained with reference tothe drawings. FIG. 2 shows a block diagram of a measurement apparatuswhich measures photoluminescence (PL) of the transparent silica glassaccording to the present invention. Methods of a laser, a detector(ICCD: image intensifier CCD) and the like as shown in FIG. 2 will bedescribed below.

FIG. 3 shows a spectrum diagram of the photoluminescence (PL) of atransparent silica glass generated at a baking temperature of 980° C.FIG. 3 reveals that the transparent silica glass generated at the bakingtemperature of 980° C. has two main peaks, a peak near a wavelength of350 nm and a peak near a wavelength of 520 nm, and further reveals thata value of the peak near the wavelength of 520 nm has a tendency toincrease with an increase in the baking time. Furthermore, the more thegenerated silica glass becomes transparent, the larger an increase rateof the peak value increasing with an increase in the baking timebecomes; and the longer the baking time becomes, the greater thetendency of the emission intensity of the photoluminescence becomes.Here, the emission intensity is normalized at photoluminescence spectrumintensity of a wavelength of 350 nm.

FIG. 3 reveals that when the baking temperature is 980° C., silicaglasses generated by baking times of 120, 144 and 168 hours aretransparent and each photoluminescence relative intensity of awavelength of 520 nm to that of a wavelength of 350 nm is apparentlygreater than photoluminescence relative intensities of silica glassesgenerated by the other baking times.

Here, although not shown in FIG. 3, when the baking temperature is 980°C., a peak value of photoluminescence of a silica glass generated by abaking time of 192 hours is lower than that of the silica glassgenerated by the baking time of 168 hours. That is, there exist optimumbaking times, in which the emission intensity becomes maximum, by everybaking temperature. For example, when the fumed silica fine particlesare pressure molded under the above-described condition and baked at thetemperature of 980° C. as shown in the present embodiment, the optimumbaking time is 168 hours. That is why the generated defect is relaxedand a defect concentration contributing to emission is reduced when thebaking time is too long.

FIG. 3 reveals that the transparent silica glass according to thepresent invention has the peak of the emission at the wavelength of 520nm and a broad emission property in which a full width at half maximum(FWHM) is approximately 200 nm in the spectrum of the photoluminescence.In actuality, when a laser beam having a weak energy density of 1 to 2mJ/cm² is irradiated to the transparent silica glass generated by thebaking temperature of 980° C. and the baking time of 168 hours, whitelight emitting arises.

Regarding a emission decay time of the transparent silica glassaccording to the present invention, wavelength lights at two main peakvalues of the photoluminescence shown in FIG. 3 are selected and livesof these are measured. The measurement results will be respectivelyexplained with reference to FIG. 4 and FIG. 5. FIG. 4 shows aphotoluminescence decay time of an emission of a wavelength of the 350nm range of the transparent silica glass generated by the bakingtemperature of 980° C. and the baking time of 168 hours. Similarly, FIG.5 shows a photoluminescence decay time of an emission of a wavelength ofthe 520 nm range. FIG. 4 shows an attenuation behavior of an emissionspecies having a fluorescent life on an order of sub-micro seconds ofapproximately 0.5 micro seconds, whereas FIG. 5 shows an attenuationbehavior of an emission species having a decay time on an order ofseveral micro seconds. This reveals that the decay time of thewavelength of 520 nm taking a leading part in broad emission wavelengthscontributing to the emission of the transparent silica glass is muchlonger than the decay times of the other wavelengths. Therefore, thewhite light emitting can be recognized by the naked eye.

FIG. 5 shows that an attenuating process of the emission is representedby two exponential functions, and a result of overlap of experiment datareveals that the attenuating process of the emission can be accuratelyrepresented by the following exponential function called a stretchedexponential function.

FIG. 10 is a graph diagram showing temperature dependency of time decayof the emission intensity at 510 nm of a sample obtained by the bakingtemperature of 980° C. and the baking time of 168 hours. Here, the solidline indicates a result of fitting with the use of the stretchedexponential function indicated in Expression 1.I=I ₀ exp(−(t/τ)^(β))   [Expression 1]

Expression 1 differs from the usual exponential function in a point thata term, β, called stretched parameter is added. When the term β is 1,the expression corresponds to the usual exponential function. Expression1 indicates that as the term β becomes smaller than 1, a distributionwidth of the emission decay time increases.

A fitting by use of the two exponential functions shown in FIG. 5 isoptimum for a case that a decay profile consists of two components.Regarding the stretched exponential function shown in FIG. 10, it isassumed that a decay time has a great number of components not only twocomponents. The term β is approximately 0.5 in the sample, and thereforeit is possible to understand that the distribution width of the emissiondecay time of the sample in the emission process is so large as to rangefrom a short decay time (several μ seconds) to a long decay time(several thousands μ seconds). A reason why the distribution width ofthe decay time is large as this is considered below.

(1) Free electrons and holes are generated in the sample by ultravioletray irradiation.

(2) The generated free electrons and holes diffuse in the sample.

(3) In a diffusion step, the free electrons recombine with the holes,consequently, the emission can be observed.

(4) A time width from the generation and diffusion to recombination ofthe free electrons and holes appears as a decay time distribution.

FIG. 11 schematically shows the above-described processes. That is, itis possible to understand that the emission process of the sample lastsfor a long time without deactivation by the cycle of each individualstep indicated below. Each step will be explained below.

Step 1 shows a process for generating the free electrons and holes withoptical excitation by the ultraviolet ray irradiation to a state thatthe defect is held without being relaxed (the state is represented bythe extended Si—O bond in FIG. 11), the state being anticipated to existin the transparent silica glass according to the present invention.Moreover, the extended Si—O bond is cut off in the process.

Step 2 a shows a process that the generated free electrons and holesrecombine with each other there to emit visible light.

Step 2 b shows a process that the generated free electrons and holesdiffuse in the sample before recombination.

Moreover, the free electrons and holes generated in Step 1 proceed toeither of Step 2 a or Step 2 b.

Step 3 shows a process that the free electrons and holes diffused inStep 2 b recombine with each other at a certain defect site to emitvisible light.

Steps 2 a, 2 b and 3 show the process that the generated free electronsand holes recombine with each other there or via the diffusion to emitvisible light.

Step 4 shows a process that a defect sight once disappears by thecut-off of the extended Si—O bond in Step 1 and appears again owing to arecovery of the Si—O bond by re-bonding of the free electrons and holes.In other words, a defect structure returns owing to the recovery of theSi—O bond by re-bonding of the free electrons and holes.

In the above-described model, as the temperature becomes lower, itbecomes difficult that the diffusion arises, and long life componentsincrease. An examination of the attenuating process of the emissionbetween liquid nitrogen temperatures of −200° C. (77 k) and 100° C. (377k) reveals that the life becomes longer as the temperature becomeslower. Such stretched exponential function attenuation of the emissionintensity regarding the silica glass has not been reported. Therefore,it is possible to understand that the emission of the sample deserves anovel emission phenomenon resulting from a specific defect stategenerated by solid phase sintering reaction of silica fine particles ofnano-size and optical excitation electrons and holes generated by thedefect state.

Embodiment 2

Emission property of a silica glass manufactured at a baking temperaturein a range of 1000 to 14000C will be explained as another embodiment.

FIG. 6 shows a spectrum diagram of the photoluminescence (PL) (silicaglasses baked at 980° C., 1000° C. and 1100° C.). FIG. 6 reveals that atransparent silica glass can be generated when the baking temperature ismore than 1000° C., the transparent silica glass having a first emissionpeak in a wavelength of 400 nm to 520 nm, a second emission peak in awavelength of 650 nm and emission property broad in the wavelength rangeof visible light in a spectrum of photoluminescence (PL).

Embodiment 3

When the silica fine particles are mixed with inorganic materialparticles having semi-conductivity and/or conductivity to be pressuremolded and baked, the defect is likely to be generated and a silicaglass having a red type emission property can be obtained. By use ofwhich, a transparent silica glass having light emitting color other thanwhite light emitting can be generated. For example, when the silica fineparticles are mixed with carbon, silicon or the like, a pink silicaglass having a red emission property is generated.

Embodiment 4

FIG. 7 shows a result of measurement of an emission excitation spectrumfor searching an optimum wavelength of an excitation light regarding theemission property of the transparent silica glass according to thepresent invention. The excitation spectrum of an emission of 510 nm ismeasured, the wavelength of 510 nm having a higher peak (thephotoluminescence intensity is higher) between the two peaks appearingin the spectra diagram (see FIG. 3) of the photoluminescence of thesample (transparent silica glass) generated by the baking temperature of980° C. and the baking time of 168 hours. FIG. 7 shows that thephotoluminescence excitation spectrum of the emission has a peak near240 nm. Since an emission intensity becomes strongest at a peak positionof an excitation spectrum, the optimum excitation wavelength of thesample is 240 nm.

Moreover, since the intensity of the photoluminescence excitationspectrum is raised at a short wavelength side from the peak, it can beexpected that even the excitation of a shorter wavelength can obtain anemission intensity equal to that of the excitation at 240 nm.

Embodiment 5

Next, a transparent silica glass that can be manufactured where theemission intensity of a short wavelength range is increased further byapplication of pre-heat treatment in the manufacturing process of thetransparent silica glass according to the present invention, will beexplained below.

In the manufacturing process of the transparent silica glass inEmbodiments 1 and 2, the pressure molding is manufactured without thepre-heat treatment of the fumed silica and the transparent silica glassis manufactured via the pre-heat treatment of the pressure molding. Inthe present embodiment, newly, the fumed silica is subjected to thepre-heat treatment at a temperature of 1000° C. and for 2 hours so thata pressure molding is manufactured by using the sample, and the pressuremolding is further subjected to the pre-heat treatment so that thetransparent silica glass is manufactured.

FIG. 8 shows an emission spectrum of the transparent silica glasssubjected to the pre-heat treatment. In FIG. 8, the curve (a) shows anemission spectrum of a sample manufactured without the pre-heattreatment and the curve (b) shows the emission spectrum of the samplemanufactured via the pre-heat treatment. FIG. 8 reveals that theemission intensity of a short wavelength component (a peak appears at350 nm) becomes higher in the emission spectrum of the transparentsilica glass subjected to the pre-heat treatment.

FIG. 8 further reveals that the conventional emission intensity near 510nm becomes about twice by the pre-heat treatment.

Embodiment 6

In addition, a pressure molding pressure to the sample is reduced andthe pressure molding is manufactured so that a transparent silica glasscan be manufactured with an increase in the emission intensity of theshort wavelength range, will be explained below.

In the manufacturing process of the transparent silica glass inEmbodiments 1 and 2, a pressure for manufacturing of the pressuremolding is fixed at 529 MPa. In the present embodiment, the pressuremolding is manufactured at a pressure of about one-thirtieth of theabove pressure, 18 MPa, so that a pressurization effect to the emissionphenomenon, a pressurization dependency of the emission phenomenon, canbe confirmed.

FIG. 9 shows an emission spectrum of the transparent silica glassmanufactured at the low pressure molding pressure. In FIG. 9, the curve(a) shows an emission spectrum of the conventional sample and the curve(b) shows a spectrum of the sample manufactured at the low pressuremolding pressure. Although a time until the silica fine particles becometransparent takes 200 hours or more, which is twice compared with a timeuntil the silica fine particles become transparent under nodecompression, FIG. 9 reveals that the emission intensity of the shortwavelength component (a peak appears at 350 nm) in the emission spectrumof the sample is high similar to the transparent silica glassmanufactured via the pre-heat treatment.

FIG. 9 further reveals that the emission intensity near 500 nm becomesabout twice by manufacturing of the sample at the low pressure moldingpressure, similar to the transparent silica glass manufacture byapplication of the pre-heat treatment.

Here, the reason why a phenomenon arises will be explained below, thephenomenon indicating that an increase in the emission intensity of theshort wavelength range by the pre-heat treatment and reduction of thepressure molding pressure respectively shown in Embodiments 5 and 6.

At first, the phenomenon will be explained that an increase in theemission intensity of the short wavelength range by the reduction of thepressure molding pressure in the manufacturing process of thetransparent silica glass.

In the pressurizing process for pressure molding the silica fineparticles and forming the pressure molding, as the pressure moldingpressure becomes lower, it is expected that a distance between particlesin the inside of the molding becomes longer. Therefore, a longer time isrequired until inter particle reaction actively arises between thesilica fine particles. Consequently, it is considered that a reactiontime until the silica fine particles become transparent becomes longerin the case of the transparent silica glass manufactured at the lowpressure molding pressure.

When the transparent silica glass is manufactured at the low pressuremolding pressure, the silica fine particles are baked at the temperatureof 1000° C. for a long time before the inter particle reaction arises.Therefore, it can be considered that most of the hydroxyl groups aresubjected to dehydration condensation reaction (on the self surfaces)before the inter particle reactions of the hydroxyl groups on theparticle surfaces arise. It is considered that the similar effect(dehydration condensation reaction on the self surfaces) can be obtainedby preheating of fumed silica as the case that the transparent silicaglass is manufactured via the pre-heat treatment. Therefore, it can beconsidered that the same emission spectrum can be obtained in the bothcases of manufacturing by application of the pre-heat treatment andmanufacturing at the low pressure molding pressure because a clearingreaction is made to arise with a low hydroxyl group concentration of thefine particle surface.

It is expected that the inter particle reaction for making the silicafine particles transparent depends on the condensation of the hydroxylgroups between the surfaces of the particles. That is, when there arefew hydroxyl groups as a reaction activity point, it becomes difficultthat a structural relaxation between the particles arises, compared tothe case that there are many hydroxyl groups. Therefore, it can beexpected that the state, in which the defect is held without beingrelaxed, can be more easily realized, the state taking a leading part ofthe emission in the transparent silica glass according to the presentinvention. Thus, the increase of the emission intensity can beexplained.

An emission peak intensity of 350 nm especially increases in Embodiments5 and 6. This indicates that a structural deformation of the defectcontributing to the emission of 350 nm is larger than that of the defectcontributing to the emission of 510 nm. That is, a relaxation state ofthe defect in the obtained transparent silica glass can be changed bycontrolling a starting concentration of the hydroxyl group on thesurface of the fine particle even if the fine particles having the samesize are used, and consequently a shape of the emission spectrum of thetransparent silica glass and the whole emission intensity can becontrolled. This is indicated in the result of Embodiments 5 and 6.

Moreover, it is confirmed that neither a change nor a degradation of theemission property of the transparent silica glass according to thepresent invention with the passage of time arises even if thetransparent silica glass is kept in the state of a usual storage statefor one year or more.

The methods of the measurement apparatuses used for measurement of thephotoluminescence (PL) of the transparent silica glass according to thepresent invention will be listed below.

-   1) Irradiation Laser Source

Pulsed Nd: YAG laser

(Spectra Physics INDI-40)

-   -   excitation wavelength: 266 nm    -   pulse width: 5-8 ns    -   repetition rate: 10 Hz    -   beam diameter <10 mm    -   laser energy: 1-2 mJ

-   2) Monochromator

Action Research SpectraPro 300i Grating

-   -   150/mm Gratings (500 nm Blaze)

-   3) Detector

ICCD

(Princeton Instruments PI-MAX 1024RB)

-   -   CCD format 1024×256 imaging pixels    -   peak QE minimum 15-20%    -   gate time 9 ns

Availability in the Industry

A transparent silica glass luminescent material according to the presentinvention is manufactured by a simple process that silica fine particlesare pressure molded and baked and has a property indicating an emissionbroad in a wavelength range of visible light, thereby can be used as aluminescent material such as a white light emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] a conceptual diagram of pressure molding of silica fineparticles

[FIG. 2] a block diagram of a measurement apparatus of photoluminescence(PL)

[FIG. 3] a spectrum diagram of the photoluminescence (PL) (a baking timeis defined as a parameter regarding a silica glass baked at atemperature of 980° C.)

[FIG. 4] a time analysis measurement diagram of a photoluminescence band(wavelength light of 350 nm)

[FIG. 5] a time analysis measurement diagram of a photoluminescence band(wavelength light of 520 nm)

[FIG. 6] a spectrum diagram of photoluminescence (silica glasses bakedat temperatures of 980° C., 1000° C. and 1100° C.)

[FIG. 7] a photoluminescence excitation spectrum diagram of a samplebaked at temperatures of 980° C. and for 168 hours (measurement bychange of a wavelength of an excitation light source during theobservation of an emission intensity of 510 nm)

[FIG. 8] an emission spectrum of a transparent silica glass subjected tothe pre-heat treatment (the curve (a) shows an emission spectrum of asample manufactured without the pre-heat treatment and the curve (b)shows the emission spectrum of the sample manufactured with the pre-heattreatment)

[FIG. 9] an emission spectrum of a transparent silica glass manufacturedat a low pressure molding pressure (the curve (a) shows an emissionspectrum of the conventional sample and the curve (b) shows a spectrumof the sample manufactured at the low pressure molding pressure)

[FIG. 10] a graph diagram showing temperature dependence of time decayof the emission intensity at 510 nm of a sample obtained by a bakingtemperature of 980° C. and a baking time of 168 hours (the solid lineindicates a result of fitting with use of a stretched exponentialfunction)

[FIG. 11] a diagram showing an emission mechanism of the transparentsilica glass

DENOTATION OF THE REFERENCE NUMBER

-   1 silica glass sample

1. A method for manufacturing a transparent silica glass luminescentmaterial comprising: a pressurizing process for pressure molding silicafine particles and forming a pressure molding; and a baking process forbaking the pressure molding under a temperature condition that astructural defect is generated and held without being relaxed.
 2. Amethod for manufacturing a transparent silica glass luminescent materialcomprising: a pressurizing process for pressure molding silica fineparticles and forming a pressure molding; and a baking process forbaking the pressure molding at a temperature in a range of 500° C. to1400° C. and in a time range of 100 minutes to 300 hours.
 3. A methodfor manufacturing a transparent silica glass luminescent materialcomprising: a pressurizing process for pressure molding silica fineparticles and forming a pressure molding; and a baking process forbaking the pressure molding at a temperature in a range of 900° C. to1000° C. and in a time range of 120 to 200 hours.
 4. A method formanufacturing a transparent silica glass luminescent material accordingto claim 1, wherein the silica fine particle is fumed silica which issynthesized by a vapor phase method and is a high-purity nano-sizesilica fine particle having a particle size in a range of 1 nm to 100nm.
 5. A method for manufacturing a transparent silica glass luminescentmaterial according to claim 1, wherein silica fine particles are mixedwith inorganic material particles having semi-conductivity and/orconductivity to be pressure molded and baked.
 6. A method formanufacturing a transparent silica glass luminescent material accordingto claim 4, further comprising a pre-heat treatment process forsubjecting fumed silica to heat treatment at 1000° C. and for 2 hoursbefore the pressurizing process for forming the pressure molding.
 7. Asilica glass luminescent material having an emission peak in awavelength of 500 nm to 520 nm and a broad emission property, in which afull width at half maximum (FWHM) is 200 nm to 300 nm, in a spectrum ofphotoluminescence (PL).
 8. A silica glass luminescent material having afirst emission peak in a wavelength of 400 nm to 520 nm, having a secondemission peak in a wavelength of 640 nm to 660 nm and indicating a broademission, in which a wavelength ranges from 300 nm to 800 nm in awavelength range of visible light, in a spectrum of photoluminescence(PL).
 9. A silica glass luminescent material, wherein the silica glassluminescent material according to claim 1 has transparency that avisible light permeation rate at a wavelength of 600 nm is not less than75%.
 10. A light emitting device, wherein a transparent silica glassluminescent material obtained by a manufacturing method according toclaim 1 is employed as a fluorescent substance.
 11. A light emittingdevice, wherein a silica glass luminescent material according to claim 7is employed as a fluorescent substance.